The present invention relates to a diversity system of the selection type for receiving and demodulating with a plurality of receiver sections a multi-phase PSK (phase shift keying) or multi-level QAM (quadrature amplitude modulation) wave carrying differentially encoded signals, and for selecting the signals with the lowest bit error rate (BER) out of the demodulated signals.
A radio digital signal transmission system usually consists of a first terminal station, a relay station and a second terminal station sequentially connected from one to another. At the first terminal station, digital signals are differentially encoded by a differential encoder and supplied to, for instance, a PSK or QAM modulator, which modulates a carrier for transmission. At the relay station, the PSK or QAM wave is received and demodulated by a demodulator, and differentially decoded by a differential decoder to recover the original digital signals. The recovered digital signals are processed in the same manner as in the aforementioned transmission from the first terminal station, and modulated and transmitted to the second terminal station, where the digital signals are recovered in the same manner as in the aforementioned reception at the relay station.
In such radio digital transmission setups, space or frequency diversity systems are generally used to increase the reliability of the transmission channel against fading or the like. Among such diversity systems, so-called selection type diversity systems are frequently used, wherein the signals with the lowest BER are selected out of baseband signals demodulated by N (.gtoreq.2, usually 2) receivers.
Such diversity systems are complex in structure since each system has pluralities of receivers, demodulators and differential decoders, the differential decoders particularly complicating the structure.
Further, whereas differential encoders and decoders are used to eliminate the phase ambiguity of the carrier wave generated by a demodulator, they may be unusable in specific cases, where for example an error-correcting MODEM (modulator/demodulator) is employed.